memblock tests: Fix mutex related build error

Fix mutex and free_reserved_area() related build errors which have
been introduced by commit 74e2498ccf ("mm/memblock: Add reserved
memory release function").

Fixes: 74e2498ccf ("mm/memblock: Add reserved memory release function")
Reported-by: Wei Yang <richard.weiyang@gmail.com>
Closes: https://lore.kernel.org/all/20250405023018.g2ae52nrz2757b3n@master/
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/174399023133.47537.7375975856054461445.stgit@devnote2
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

tools/testing/memblock/linux/mutex.h

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _MUTEX_H
+#define _MUTEX_H
+
+#define DEFINE_MUTEX(name) int name
+
+static inline void dummy_mutex_guard(int *name)
+{
+}
+
+#define guard(mutex)	\
+	dummy_##mutex##_guard
+
+#endif /* _MUTEX_H */